TY  - RPRT
TI  - Letter Circular 680: incandescent electrical lamps: photometric standards distributed by the National Bureau of Standards (superseding Letter Circular 220)
PY  - 1942
PB  - National Institute of Standards and Technology
CY  - Gaithersburg, MD
SN  - LCIRC 680
DO  - 10.6028/NBS.LCIRC.680
ER  -